Optical glass

ABSTRACT

Provided is an optical glass having a high refractive index and high light transmittance. The optical glass contains, in terms of mol % based on oxides, SiO 2 : 9.0% to 11.0%, B 2 O 3 : 22.0% to 24.0%, La 2 O 3 : 18.0% to 20.0%, and TiO 2 : 30.0% to 31.0%.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of International Application No. PCT/JP2019/033347, filed on Aug. 26, 2019, the entire contents of which are incorporated herein by reference.

FIELD

The present invention relates to an optical glass, and in particular relates to an optical glass to be used for a light guide plate.

BACKGROUND

In recent years, head mounted displays (HMD) compatible with, for example, augmented reality (AR), virtual reality (VR), and mixed reality (MR) have been a topic of interest. As a light guide plate for this optical device, it is known to use a glass having a high refractive index. For a light guide plate for HMD, the thickness of the light guide plate and a refractive index, which greatly affects flexibility in the total design of HMD, are particularly important. For example, Patent Literature 1 discloses a high-refractive-index glass for HMD, the glass having a refractive index of approximately 2.0.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Laid-open Patent Publication No. 2019-20723

SUMMARY Technical Problem

However, in recent years, higher performance has been required for products, and accordingly, higher light transmittance of glass has been required. The optical glass described in Patent Literature 1 has an insufficient internal transmittance, that is, approximately 0.6 in the blue wavelength range.

Under such circumstances, an optical glass having a high refractive index and high light transmittance has been desired.

Solution to Problem

An optical glass of the present invention comprises, in terms of mol % based on oxides:

SiO₂: 9.0% to 11.0%;

B₂O₃: 22.0% to 24.0%;

La₂O₃: 18.0% to 20.0%; and

TiO₂: 30.0% to 31.0%.

Advantageous Effects of Invention

According to the present invention, an optical glass having a high refractive index and high light transmittance can be industrially produced. In particular, the present invention is suitable for continuous production.

DESCRIPTION OF EMBODIMENTS

Descriptions are given of an optical glass of the present invention. The shape of the optical glass is not particularly limited. For example, the optical glass may be shaped like a plate.

[Component of Optical Glass]

The optical glass of the present invention includes a combination of essential components and optional components. The essential components are essentially contained in the optical glass and offer important functions in performance. The optional components are used if needed. Unless there is a specific description in the present specification, % means mol % based on oxides. A numerical range includes a rounded-of range. A numerical range “A to B” means “A or more and B or less”.

[Essential Component]

<SiO₂>

SiO₂ is a component that forms glass, gives strength and cracking resistance to glass, and enhances the stability and chemical durability of glass. The optical glass of the present embodiment has a SiO₂ content of 9.0% to 11.0%, and preferably 9.0% to 10.0%. The optical glass having a SiO₂ content within the above-mentioned range can acquire both high strength and a high refractive index.

<B₂O₃>

B₂O₃ is a component that provides a low glass transition temperature (Tg) and enhances mechanical properties such as the strength and the crack resistance of glass. The optical glass of the present embodiment has a B₂O₃ content of 22.0% to 24.0%, and preferably 22.5% to 23.5%. The optical glass having a B₂O₃ content within the above-mentioned range can acquire both high strength and a high refractive index.

<La₂O₃>

La₂O₃ is a component that contributes to increasing the refractive index of the present optical glass and lowering the devitrification temperature of the present optical glass. The optical glass of the present embodiment has a La₂O₃ content of 18.0% to 20.0%, and preferably 18.5% to 19.5%. The optical glass having a La₂O₃ content within the above-mentioned range can acquire both a high refractive index and a low devitrification temperature.

<TiO₂>

TiO₂ is a component that, typically, increases the refractive index of glass and enlarges the dispersion of glass, in the present optical glass, TiO₂ is a component that contributes to, in particular, the devitrification temperature and light transmittance of the optical glass. The optical glass of the present embodiment has a TiO₂ content of 30.0% to 31.0%, and preferably 30.3% to 30.7%. The optical glass having a TiO₂ content within the above-mentioned range can acquire both a high transmittance and a low devitrification temperature.

[Optional Component]

<Y₂O₃>

The optical glass of the present embodiment preferably contains Y₂O₃. Y₂O₃ is a component that contributes to increasing the refractive index of the present optical glass and lowering the devitrification temperature of the present optical glass. In the case where the optical glass of the present embodiment contains Y₂O₃, the Y₂O₃ content of the optical glass is preferably 3.0% to 5.0%, and particularly preferably 3.5% to 4.5%. The optical glass having a Y₂O₃ content within the above-mentioned range can acquire both a high refractive index and a low devitrification temperature.

<WO₃>

The optical glass of the present embodiment preferably contains WO₃. WO₃ is a component that inhibits the devitrification of glass and contributes to the coloring of glass. Therefore, a too large amount of WO₃ causes a decrease in light transmittance. Hence, in the case where the optical glass of the present embodiment contains WO₃, the WO₃ content of the optical glass is preferably 0.1% to 0.4%, and particularly preferably 0.2% to 0.3%. The optical glass having a WO₃ content within the above-mentioned range can acquire both a high transmittance and a low devitrification temperature.

<Nb₂O₅>

The optical glass of the present embodiment preferably contains Nb₂O₅. Nb₂O₅ is a component that contributes to increasing the refractive index of the present optical glass and lowering the devitrification temperature of the present optical glass. In the case where the optical glass of the present embodiment contains Nb₂O₅, the Nb₂O₅ content of the optical glass is preferably 2.5% to 4.0%, and particularly preferably 3.0% to 3.5%. The optical glass having a Nb₂O₅ content within the above-mentioned range can acquire both a high refractive index and a low devitrification temperature.

<ZrO₂>

The optical glass of the present embodiment preferably contains ZrO₂. ZrO₂ is a component that increases the refractive index of glass and enhances the chemical durability of glass. In the case where the optical glass of the present embodiment contains ZrO₂, the ZrO₂ content of the optical glass is preferably 5.0% to 8.0%, and particularly preferably 6.0% to 7.0%. The optical glass having a ZrO₂ content within the above-mentioned range can acquire both high durability and a high refractive index.

<Gd₂O₃>

The optical glass of the present embodiment preferably contains Gd₂O₃. Gd₂O₃ is a component that contributes to increasing the refractive index of the present optical glass and lowering the devitrification temperature of the present optical glass. In the case where the optical glass of the present embodiment contains Gd₂O₃, the Gd₂O₃ content of the optical glass is preferably 3.0% to 5.0%, and particularly preferably 3.5% to 4.5%. The optical glass having a Gd₂O₃ content within the above-mentioned range can acquire both a high refractive index and a low devitrification temperature.

<Other Optional Components>

Besides the above-mentioned components, the optical glass of the present embodiment may include a minor component and an additive that are typically used for the production of optical glasses, without impairing the effects of the present invention.

[Composition of Optical Glass]

The optical glass of the present embodiment contains the following compounds in mol % based on oxides.

SiO₂: 9.0% to 11.0%

B₂O₃: 22.0% to 24.0%

La₂O₃: 18.0% to 20.0%

TiO₂: 30.0% to 31.0%

The optical glass of the present embodiment preferably contains the following compounds in mol % based on oxides.

SiO₂: 9.0% to 11.0%

B₂O₃: 22.0% to 24.0%

Y₂O₃: 3.0% to 5.0%

TiO₂: 30.0% to 31.0%

WO₃: 0.1% to 0.4%

Nb₂O₅: 2.5% to 4.0%

La₂O₃: 18.0% to 20.0%

ZrO₂: 5.0% to 8.0%

Gd₂O₃: 3.0% to 5.0%

The optical glass of the present embodiment particularly preferably contains the following compounds in mol % based on oxides.

SiO₂: 9.0% to 10.0%

B₂O₃: 22.5% to 23.5%

Y₂O₃: 3.5% to 4.5%

TiO₂: 30.3% to 30.7%

WO₃: 0.2% to 0.3%

Nb₂O₅: 3.0% to 3.5%

La₂O₃: 18.5% to 19.5%

ZrO₂: 6.0% to 7.0%

Gd₂O₃: 3.5% to 4.5%

Furthermore, the composition of the optical glass of the present embodiment is preferably in a range of 99.5%≤(SiO₂+B₂O₃+Y₂O₃+TiO₂+WO₃+Nb₂O₅+La₂O₃+ZrO₂+Gd₂O₃)≤100.0%, and particularly preferably (SiO₂+B₂O₃+Y₂O₃+TiO₂+WO₃+Nb₂O₅+La₂O₃+ZrO₂+Gd₂O₃)=100.0%, in mol % based on oxides (note that the inclusion of inevitable impurities due to manufacture is allowed).

The optical glass having a composition within the above-mentioned range can satisfy all of a high refractive index, light transmittance, and a low devitrification temperature.

Furthermore, the optical glass of the present embodiment preferably does not contain ZnO in an amount larger than the amount of impurities inevitably contained due to manufacture. If containing ZnO, the present optical glass is easily devitrified.

Furthermore, the optical glass of the present embodiment preferably does not contain alkali metal in an amount larger than the amount of impurities inevitably contained due to manufacture. If containing alkali metal, the present optical glass has lower weather resistance and lower chemical resistance.

[Optical Glass]

The optical glass of the present embodiment has a refractive index nd of preferably 1.90 to 2.10, and particularly preferably 1.95 to 2.05. In particular, in the case where the optical glass of the present invention is used for a light guide plate, the optical glass having a refractive index nd in the above-mentioned range makes it possible to design a thinner light guide plate.

The optical glass of the present embodiment has an internal transmittance at a wavelength of 450 nm of preferably 88.0% or higher, and particularly preferably 90.0% or higher. The optical glass having an internal transmittance in the above-mentioned range can substantially prevent the attenuation of blue light in a light guide plate, and can substantially prevent a picture projected on HMD from looking yellow.

The optical glass of the present embodiment has a devitrification temperature of preferably less than 1,170° C., and particularly preferably less than 1,150° C. The optical glass having a devitrification temperature in the above-mentioned range reduces devitrification caused by crystal formation during glass manufacture, and thereby makes glass manufacture easier.

[Method for Producing Optical Glass]

A method for producing the optical glass of the present embodiment is not limited to a particular method, and existing methods for producing plate glasses can be applied. For example, well-known methods, such as float method, fusion method, and roll-out method, can be used.

EXAMPLES

Hereinafter, detailed descriptions will be given of Examples and Comparative Examples of the present invention. As long as the advantageous effects of the present invention are exhibited, the embodiments may be suitably modified.

<Measurement Method>

[Devitrification Temperature]

A devitrification temperatures T of the optical glass of each of Examples and Comparative Examples was measured on the following conditions.

Raw materials of compositions listed in Table 1 were melted in a platinum crucible at 1,250° C. for 2 hours to form a uniform molten glass. The molten glass was casted into a mold (length×width×height=60 mm×50 mm×30 mm) heated to 200° C. to obtain a glass block. The glass block was put into water of ordinary temperature to be pulverized, so that cullet measuring approximately 2 mm per side was obtained. From this cullet, 5 g of cullet was taken as an evaluation sample.

The evaluation sample was put into a platinum plate, and heated for 16 hours in an electric furnace set at a predetermined temperature of 1,100° C. to 1,200° C. to melt the sample. After a lapse of the 16 hours, the sample was taken out of the electric furnace, and, while the sample was naturally allowed to cool, whether the sample was precipitated as crystals was observed by using an optical microscope. Conditions on which crystals had been observed were recorded, and the highest temperature among set temperatures of the electric furnace under the conditions is regarded as a devitrification temperature T. For the optical glasses of Examples and Comparative Examples, devitrification temperatures T of evaluation samples are judged on the following criteria, and it is determined that a devitrification temperature T with a double circle or a circle is acceptable.

Double Circle: T<1,150° C.

Circle: 1,150≤T<1,170° C.

Cross: 1,170° C.≤T

[Light Transmittance]

A light transmittance X was measured on the following conditions.

Two types of glass plates were used, obtained in Examples and the likes and having a size of length×width=30 mm×30 mm, one type of the glass plates having a plate thickness of 10 mm and the other type of the glass plates having a plate thickness of 1 mm. The light transmittance X at a wavelength of 450 nm of a glass plate having a plate thickness of 10 mm was measured by a spectrophotometer (U-4100, manufactured by Hitachi High-Technologies Corporation) (the light transmittance in the present embodiment means an internal transmittance except a reflection loss in front and back faces). For the optical glasses of Examples and Comparative Examples, the light transmittance X of each of the evaluation samples is judged on the following criteria, and it is determined that a light transmittance X marked with a double circle or a circle is acceptable.

Double Circle: 90.0%≤X

Circle: 88.0%≤X<90.0%

Cross: 88.0<X

[Refractive Index]

The refractive index (nd) of each of the glass plates obtained in Examples and the likes and having a size of length×width×plate thickness=30 mm×30 mm×10 mm was measured by a refractometer (KPR-2000, manufactured by Kalnew).

Example 1

A raw material of a composition listed in Table 1 was melted in a platinum crucible at 1,250° C. for 2 hours to obtain a uniform molten glass. The molten glass was poured into a mold (length×width×height=60 mm×50 mm×30 mm) heated to 200° C. to obtain a glass block.

Next, by the use of a cutting machine (a small-sized cutting machine manufactured by Maruto Instrument Co., Ltd.), the glass block was cut into pieces each having a size of length×width=30 mm×30 mm, and the resulting pieces were subjected to plate thickness adjustment and surface polishing by the use of a grinding machine (SGM-6301, manufactured by SHUWA Industry Company Limited) and an one-side polishing machine (EJ-380IN, manufactured by Engis Japan Corporation) to produce glass plates having a size of length×width=30 mm×30 mm and a plate thickness of 10 mm or 1 mm. These glass plates were subjected to various evaluations. Table 1 lists evaluation results.

Grinding Condition: The glass plates were ground at 2.0 μm/sec by the use of a #100 diamond wheel, and then ground at 1.0 μm/sec by the use of a #1000 diamond wheel, and subsequently ground at 0.5 μm/sec by the use of a #2000 diamond wheel.

Polishing Condition: abrasive (cerium oxide), the number of revolutions (80 rpm for 10 minutes)

Examples 2 to 3, Comparative Examples 1 to 2

The same operation as in Example 1 was performed, except that the condition was changed to conditions listed in Table 1.

TABLE 1 Evaluation Devitrification Light Composition (mol %) Temperature Transmittance SiO₂ B₂O₃ Y₂O₃ TiO₂ WO₃ Nb₂O₅ La₂O₃ ZrO₂ Gd₂O₃ nd ° C. Judgment % Judgment Example 1 9.5 23.1 4.0 30.5 0.25 3.3 19.0 6.5 3.9 2.00 1138 ⊚ 92.2 ⊚ Example 2 9.8 23.1 3.7 31.0 0.25 3.2 19.0 6.4 3.7 2.00 1125 ⊚ 88.7 ◯ Example 3 9.3 23.1 4.3 30.0 0.25 3.4 19.1 6.5 4.1 2.00 1155 ◯ 92.8 ⊚ Comparative 10.0 23.1 3.4 31.5 0.25 3.1 18.9 6.3 3.5 2.00 1113 ⊚ 87.0 X Example 1 Comparative 9.0 23.1 4.6 29.5 0.25 3.5 19.1 6.6 4.4 2.00 1175 X 92.3 ⊚ Example 2 

1. An optical glass comprising, in terms of mol % based on oxides: SiO₂: 9.0% to 11.0%; B₂O₃: 22.0% to 24.0%; La₂O₃: 18.0% to 20.0%; and TiO₂: 30.0% to 31.0%.
 2. The optical glass according to claim 1, further comprising, in terms of mol % based on oxides: Y₂O₃: 3.0% to 5.0%; WO₃: 0.1% to 0.4%; Nb₂O₅: 2.5% to 4.0%; ZrO₂: 5.0% to 8.0%; and Gd₂O₃: 3.0% to 5.0%.
 3. The optical glass according to claim 2, satisfying, in terms of mol % based on oxides: SiO₂: 9.0% to 10.0%; B₂O₃: 22.5% to 23.5%; Y₂O₃: 3.5% to 4.5%; TiO₂: 30.3% to 30.7%; WO₃: 0.2% to 0.3%; Nb₂O₅: 3.0% to 3.5%; La₂O₃: 18.5% to 19.5%; ZrO₂: 6.0% to 7.0%; and Gd₂O₃: 3.5% to 4.5%.
 4. The optical glass according to claim 1, having a composition in a range of 99.5%≤(SiO₂+B₂O₃+Y₂O₃+TiO₂+WO₃+Nb₂O₅+La₂O₃+ZrO₂+Gd₂O₃)≤100.0%, in terms of mol % based on oxides.
 5. The optical glass according to claim 1, having a refractive index in a range of 1.90 to 2.10.
 6. The optical glass according to claim 1, having an internal transmittance at 450 nm of 88.0% or higher.
 7. The optical glass according to claim 1, having a devitrification temperature T in a range of T<1,170° C. 